Radio frequency identification concepts and technology date back nearly 80 years, and since then, RFID principles have been used in a wide variety of automatic identification applications. The same principles of inductively coupled power and data have been used in related fields such as medical and industrial telemetry, IC cards for financial transactions and security access, anti-shoplifting systems, tollway payment systems, and inventory tracking. RFID used for animal identification was pioneered at The Los Alamos Scientific Laboratory in the late 1970's. In the 1980's, RFID animal identification systems became commercially viable, and several suppliers developed similar, but mutually incompatible, products for tagging animals and reading identification codes.
Government interest in RFID animal identification developed in the late 1980's with the formation of the European Union and ensuing concerns about potentially uncontrolled transport of animals across international borders. In response, the International Standards Organization was chartered in the early 1990's with developing an international standard based on one or more of the existing RFID technologies, with the intent of identifying all livestock with RFID tags. A practicable and operable international identification system requires a standard RFID system, rather than a proliferation of the multiple mutually incompatible proprietary systems. After several years of research, investigation, and negotiation, ISO published Standards 11784 and 11785.
The multitude of mutually incompatible RFID systems has, however, remained, and has even expanded with the introduction of systems operating in other frequency bands, such as 13.56 MHz, for example. Suppliers of RFID components have responded by providing integrated circuits (ICs) and interrogators that are capable of operability in compliance with these various mutually incompatible systems. For example, many commercially available RFID transponder ICs can be programmed with any identification code, and can be configured to be compatible with an ISO interrogator or with any one of several proprietary interrogators. Several transponder ICs from manufacturers such as Atmel Corporation of San Jose, Calif., EM Microelectronic—US, Inc. of Colorado Springs, Colo. and NXP Semiconductors Netherlands B.V., of Eindhoven, Holland illustrate such configurable devices, and numerous patents including U.S. Pat. No. 5,218,343 to Stobbe, et al., U.S. Pat. No. 5,053,774 to Schuermann, et al., U.S. Pat. No. 5,602,744 to Meek, et al., U.S. Pat. No. 5,349,649 to Iijima, U.S. Pat. No. 5,302,964 to Brooks, et al., U.S. Pat. No. 5,530,232 to Taylor, and U.S. Pat. No. 6,249,212 to Beigel, et al. track this evolution.
ISO Standard 11785 defines the technical principles for communications between interrogators (alternately referred to as “readers” or “scanners”) and two types of electronic passive identification transponders, and ISO 11784 defines the allocation of transponder memory bits for identification purposes. These transponders contain identification information stored in binary form, which is conveyed to the interrogator when a transponder is suitably activated by the interrogator. Additional technical details are provided in ISO Standards 11784 and 11785, the disclosure of which is incorporated into this disclosure by reference in its entirety.
ISO 11784 and 11785 relate to radio frequency identification (RFID) systems, comprising low frequency interrogator devices and passive inductively powered identification tags. In RFID systems of this type, the interrogator generates a tag activation signal, and receives an identification data signal from the ID tag. ISO 11785 specifies a periodic activation signal (a burst-mode carrier) which prompts an FDX-B tag to respond during the interval in which the carrier is present, and which prompts an HDX tag to respond during the interval in which the carrier is absent.
Passive low frequency RFID interrogators and tags use operating principles that are well-know to those of ordinary skill in the art, and that are described in extensive detail in several seminal inventions, including U.S. Pat. No. 1,744,036 (Brard—1930), U.S. Pat. No. 3,299,424 (Vinding—1967), U.S. Pat. No. 3,713,146 (Cardullo—1973), and U.S. Pat. No. 5,053,774 (Schuermann—1991), and in textbooks such as “RFID Handbook” (Finkenzeller—1999).
As depicted in FIG. 1, the interrogator 100 includes electronic circuitry, which generates an activation signal (usually a single frequency unmodulated sinusoidal signal) using a signal source 101 and an amplifier 102 to drive a resonant antenna circuit 103. This activation signal manifests as a time-varying electromagnetic field, which couples with the ID tag 105 by means of the electromagnetic field's magnetic field component 104. The ID tag 105 converts this magnetic field into an electrical voltage and current, and uses this electrical power to activate its internal electronic circuitry. Using any of several possible modulation schemes, the ID tag conveys binary encoded information stored within it back to the interrogator via magnetic field 104, where the detector and utilization circuit 106 convert this binary code into typically decimal, hexadecimal, or alphanumeric format tag data 107 in accordance with some prescribed application.
ISO Standard 11785 defines two types of transponder technologies, which are designated “full-duplex” (“FDX-B”) and “half-duplex” (“HDX”). In the described manners that follow, for HDX and FDX-B transponders, respectively, activation energy is transferred to the transponder from the interrogator, and identification code information is transferred to the interrogator from the transponder through the mutual coupling of a magnetic field.
The FDX-B transponder amplitude modulates the interrogator's activation signal with its binary identification code sequence. The interrogator detects this modulation and derives from it the FDX-B transponder's identification code. The term “full-duplex” derives from the FDX-B transponder's behavior wherein its identification code information is transmitted simultaneously during receipt of the activation signal from the interrogator.
In contrast, the HDX transponder uses the interrogator's activation signal to charge an internal capacitor (which functions as a very small rechargeable battery), and it uses this stored energy to activate a transmitter, which emits a frequency shift keyed (“FSK”) signal representative of the transponder's identification code. The interrogator detects this FSK signal and derives from it the HDX transponder's identification code. The term “half-duplex” derives from the HDX transponder's behavior wherein the exchange of the activation signal and the identification code signal occur during alternate time intervals.
An ISO 11785 compliant interrogator has the capability to activate and detect both FDX-B and HDX type transponders. To accomplish this, the ISO compliant interrogator transmits an activation signal, consisting of a 134.2 kilohertz (KHz) sinusoid, which is switched ON and OFF in a prescribed cadence in accordance with ISO 11785. During the interval in which the 134.2 KHz carrier is ON, the FDX-B transponder is activated and it transmits its identification code signal cyclically for as long as the carrier signal is present. During this ON interval also, an HDX transponder charges it internal capacitor. Subsequently, during the interval in which the 134.2 KHz carrier signal is OFF, the FDX-B transponder is dormant, and the HDX transponder transmits its identification code sequence once.
The FDX-B transponder communicates to the interrogator by amplitude modulating the activation signal it receives. Amplitude modulation imposes variations on the activation signal's magnitude, and the interrogator is equipped with sensing circuitry capable of detecting these magnitude variations. This reflected signaling is somewhat analogous to shining a light beam on a distant mirrored surface and inducing a motion to the mirrored surface, which varies the amount of light that is reflected back to the light source.
An HDX transponder, in contrast, contains its own micro-transmitting capability, which is powered with energy received and stored from the interrogator's activation signal. Once the activation signal ceases, the HDX transponder emits a very small strength internally generated radio signal, comprising a frequency shift keyed (“FSK”) modulation scheme. Specifically, the binary identification code information contained in the HDX tag is serially output such that the occurrence of a binary “1” results in the HDX tag's radio signal being 124.2 KHz and a binary “0” results in the tag's radio signal being 134.2 KHz. Thus, a corresponding analogy for HDX might include a light source that briefly illuminates a solar cell that charges a battery. When the light source extinguishes, the charge stored in the battery is used to alternately illuminate a red light source and a blue light source in accordance with some prescribed sequence.
FIG. 2a provides a block diagram illustration of an ISO identification tag. Although FDX-B and HDX transponders have different internal circuit designs supportive of their respective behaviors, the operation of both can be described using this generic block diagram. For simplicity, the powering circuitry is omitted.
Referring to the block diagram of FIG. 2a, and to the FDX-B waveforms of FIG. 2b, an FDX-B tag receives an activation signal which manifests as a 134.2 KHz sinusoidal voltage FO 204 illustrated in FIG. 2b-1 across the terminals 205a, 205b of the Resonant Antenna Circuit 201, comprising antenna L 202 and capacitor C 203. A portion of this voltage is converted to direct current (DC) and is used to power the tag's circuitry 200. Another portion of the sinusoidal voltage FO 204 is converted to a digital pulse signal F1 207 by Clock Generator 206 and is used to clock the tag's digital circuitry. The output of the clock generator, F1 207 is reduced by the Frequency Divider 208 by a factor of 32 to produce signal FBR 209 of approximately 4194 Hz. This signal frequency establishes the bit rate of the tag, and it is used to clock data out of the ID Code Memory 210, wherein the data resides as a sequence of binary 1's and 0's as is illustrated in FIGS. 2b-2, and 2b-3. In other words, for every 32 input pulses of the digital pulse signal F1 207, a new identification code data bit (FIGS. 2b-2, 2b-3) is output from the ID Code Memory 210.
As the binary 1's and 0's 211 are output from the ID Code Memory 210, they pass through an Encoder 212, which bi-phase encodes the data. This transformation converts a binary 1 into a binary 1 or a binary 0 having a full bit width duration, and converts a binary 0 into a binary 1/0 or 0/1 pair having a full bit width duration (see FIG. 2b-4). The output 213 of the Encoder 212 is applied to the Modulation Switch SM 215, which opens and closes in response to binary 0's and 1's, respectively, from the Encoder signal 213. With switch SM 215 closed, Load Impedance ZM 216 is connected across the Resonant Antenna Circuit 201, which has the effect of attenuating the amplitude of sinusoidal signal FO 204. This results in an amplitude modulated signal, such as the amplitude modulated signal illustrated in FIG. 2b-5. This amplitude modulation is detected by the interrogator, and is converted back to binary 1's and 0's and thereby the identification code information contained in the tag's ID Code Memory 210 is recovered.
Referring again to the block diagram of FIG. 2a, and to the HDX waveforms of FIG. 2c, an HDX tag receives an activation signal from the interrogator which manifests as a 134.2 KHz sinusoidal voltage FO 204 illustrated in FIG. 2c-1 appearing across the terminals 205a, 205b of the Resonant Antenna Circuit 201. This voltage is converted to direct current and powers a portion of tag circuitry 200 that controls the accumulation of electrical charge in a capacitor (not shown) and also holds the tag in a suspended communication state. When the HDX tag power control circuitry (not shown) has detected that the sinusoidal voltage FO has diminished in amplitude, the HDX tag enters its transmission active state.
The Clock Generator 206 in the HDX tag, in conjunction with the Resonant Antenna Circuit 201 includes a ringing oscillator, that continues to oscillate at its natural frequency, (which is approximately the same as the activation signal frequency FO), when FO 204 ceases. A ringing oscillator operates in a manner very much like a musical instrument's string, which is periodically plucked so that is remains oscillating. Such a ringing oscillator is disclosed in U.S. Pat. No. 3,995,234, the disclosure of which is incorporated herein by reference in its entirety. The oscillator output F1 207 is applied to the frequency divider which reduces F1 207 by a factor of 16, which in turn becomes signal FBR 209 having approximate frequency 8387 Hz. This frequency establishes the bit rate of the tag, and it is used to clock Binary Data 211 out of the ID Code Memory 210, wherein the Binary Data 211 resides as a sequence of binary 1's and 0's (see for example FIGS. 2c-2, 2c-3). In other words, for every 16 input pulses of F1 207, a new identification code Binary Data bit is output from the ID Code Memory 210.
Binary 1's and 0's, such as the binary data illustrated in FIGS. 2c-2, 2c-3, are clocked out of the ID Code Memory 210 and bypass the Encoder 212 so that Binary Data 211, 213 is applied directly to Modulation Switch SM 215. Switch SM 215 opens and closes in response to the binary 0's and 1's, respectively. In an HDX transponder, Load Impedance ZM 216 is typically a capacitive element that is connected across the Resonant Antenna 201 when switch SM 215 closes in response to a binary 1. This capacitor ZM 216 has the effect of altering the effective resonant frequency of the Resonant Antenna 201 thereby altering the operating frequency of the ringing oscillator to 124.2 KHz. Consequently, the oscillator output F1 207 becomes 124.2 KHz, which is reduced by a factor of 16 by the Frequency Divider 208 to produce the signal FBR 209 having the approximate frequency 7762 Hz. As shown in FIG. 2c-4, the ringing oscillator changes its frequency between 134.2 KHz and 124.2 KHz in response to binary 0's and 1's, thus creating a frequency shift keyed (FSK) sinusoidal signal (see for example FIG. 2c-4) that appears across the resonant antenna circuit 201. As can be seen from FIG. 2c-4, the period of a binary 1 is greater (about 129 usec) than the period of a binary 0 (about 119 usec), since the bit rate is determined by dividing the ringing oscillator's instantaneous frequency (either 134.2 KHz or 124.2 KHz) by 16.
FIGS. 3a and 3b illustrate the frequency spectral characteristics of the RFID system pertaining to ISO 11785 and to the present invention. FIG. 3a shows the spectra for the HDX tag, where the activation signal 301 appears at 134.2 KHz, and where the HDX transponder frequencies appear at 124.2 KHz 302a and 134.2 KHz 302b. Since the activation signal 301 and the HDX transponder signals 302a, 302b alternate in time, the 134.2 KHz activation signal 301 and the 134.2 KHz transponder signal 302a, 302b do not occur simultaneously. Thus, the interrogator's receive circuitry is able to detect the transponder data signal without being interfered with by its own activation signal.
FIG. 3b shows the spectra for the FDX-B tag, where the activation signal 303 appears at 134.2 KHz, and where the FDX-B transponder's amplitude modulation appears as sidebands close to the 134.2 KHz 304a, 304b. As is known to those of ordinary skill in the art, amplitude modulation sidebands appear symmetrically around the modulated carrier signal, and for FDX-B specifically, these sidebands appear at ±2.097 KHz and ±4.194 KHz. Because the activation signal 303 frequency and the data signal 304a, 304b frequencies are distinct, they can occur simultaneously, and the interrogator is able to separate them, thereby recovering the data contained therein. Removal of the 134.2 KHz carrier through envelope detection results in the translation of these data frequencies as shown in FIG. 3c. As can be observed, HDX 305a, 305b and FDX-B 306 tag signal spectra occupy different frequency bands 307, 308, and are thus frequency diverse.
Referring to FIG. 4, ISO 11785 also specifies a periodic activation signal having an adaptive timing characteristic that depends on the interrogator's instantaneous detection of an HDX or an FDX-B tag, the purpose of which is to increase the effective interrogation rate, and thereby improve reading speed. Specifically, this adaptive timing scheme requires the following:                [FIG. 4a] The interrogator activation signal default cycle time is 50 msec ON (401) and 3 msec OFF (402) when neither FDX-B nor HDX transponders are being read, but are being searched for.        [FIG. 4b] If, during the 3 msec OFF interval (402), an HDX transponder signal is sensed by the interrogator, the interrogator extends the OFF interval (402) to 20 msec (403) in order to completely capture and read the HDX transponder's identification code.        [FIG. 4c] If, during the 50 msec ON interval (401), an FDX-B transponder signal is sensed by the interrogator, the interrogator may extend the ON interval (401) up to 100 msec (404), if necessary, to completely capture and read the FDX-B transponder's identification code.        [FIG. 4d] If both tags are sensed during their respective transmission intervals (401, 402) the OFF interval (402) is extended to 20 msec, (405) and the ON interval (401) may be extended up to 100 msec (406).        
In addition, every tenth activation signal cycle has ON and OFF intervals fixed at 50 msec ON and 20 msec OFF, regardless of transponder sensing and reading status. Thus, an interrogator may exhibit four activation signal ON/OFF cadences:                a) No tag being sensed: 50 msec ON/3 msec OFF        b) HDX tag sensed and read: 50 msec ON/20 msec OFF        c) FDX-B tag sensed and read: 50 to 100 msec ON/3 msec OFF        d) FDX-B and HDX tags sensed and read: 50 to 100 msec ON/20 msec OFF        